Apparatus for producing hardened optical coatings by electron bombardment



Oct. 14,' 1947. G. l.. DIMMICK APPARATUS FOR PRODUCING HARDENED OPTICAL COATINGS BY ELECTRON BOIBARDMENT Filed May 1, 1944 (Ittomcg i W 0oz arramzmm Oct. 14, 1947. G, L, D|MM|K 2,428,868

APPARATUS Fon PnoDUcING HARDENED OPTICAL GOATINGS BY ELECTRON BOMBARDMENT Filed lay 1, 1944 2 Sheets-Sheet 2 Zmventor g1g/messa 1 P A a,

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APatented Oct. 14, 1947 UNITED STATES' PATENT. orifice APPARATUS FOR PRODUCING HiIRII'INIIDA OPTICAL COATINGS BY ELECTRON BOM- BARDIVIENT Glenn L. Dimmick, Indianapolis, Ind., assigner l to Radio ,Corporation of America, a corporation of Delaware Application May 1, 1944, Serial No. 533,526

4 Claims.

,` apparatus for producing a light-transparent film on the surface ci' an optical object, characterized by substantially improved hardness and durability, in a relatively short processing time tothereby substantially speed up production and lower costs. A

Another object of the invention is to provide apparatus for substantially improving the hardness, durability and chemical stability of a thermally evaporated film on a. surface of an optical article without deleteriously affecting said article. as by thermal eifects.

Another object of the invention is to provide an improved apparatus for treating a iilm having reflection-reducing characteristics on an optical surface to make the illm more compact.

Still another object of the invention is to provide apparatus forl producing a thin protective coating of improved hardness and durability on front surface optical mirrors, without deleteriously affecting the optical properties of the mirror.

Further additional objects and advantages not heretofore speciiied will become apparent from the following detailed description.

Referring to the drawings:

Figure 1 is a side elevation view of a bell jar and enclosed apparatus embodying the invention,

Figure 2 is a schematic view, partly in section, of certain of the apparatus shown in Figure 1 and including a diagram of the electrical circuit,

Figure 3 is a plan view of an optical object treated in accordance with the invention,

Figures 4, 5. 6, and 'I are characteristic curves involving the apparatus in accordance with the invention,

Figure 8 is a side elevation view of vacuum apparatus embodying a preferred form of the invention,

Figure 9 is a side elevation view in section of an electron gun, shown schematically in Figure 8 in miniature, approximately half size, and

Figure 10 is a circuit diagram of the scanning system for the electron gun in Figures 8 and 9.

In accordance with the invention, the hardness, toughness, durability and chemical stability of a thermally evaporated illm on an opticalarticle is substantially improved, without deleteriously affecting the optical or physical characteristics of said article, by treating said illm, preferably magnesium fluoride. MgFz, with an electrical discharge, preferably electron bombardment, during or after the thermal evaporation process in vacuo. The coating or film is hardened by bombardment with particles, Aa term used to include molecules, atoms, ions, or electrons which'make the coating more dense and compact. Heretofore, such coatings have been hardened by means of baking at high temperatures .after removing the article from the bell jar or by means of the application of radiant heat in the bell jar prior to the film-forming process. While these processes have substantially improved the durable characteristics of the lm, the heat involved has been a problem in regard to the optical body, particularly cemented duplex lenses. The heating has set up objectionable strains in high precision optical bodies. Furthermore, the time required for heating and cooling such bodies has slowed production. According to the present invention, the lm is treated in such a manner as to substantially improve the hardness and durability of the lm without deleteriously affecting the cement between doubletlenses and without producing strains of such magnitude as to harm precision optical bodies.

It has been observed that when most materials having a lower index of refraction than about 1.5 are evaporated onto glass in a vacuum at room temperatures, the reflectivity reaches a minimum value which is somewhat lower than 'is realized after air containing moisture is allowed to fill the bell jar.. For instance, when magnesium iluoride is evaporated onto spectacle crown glass at room temperatures, the minimum reflectivity is about 15 per cent of its former value. When moist air is let into the jar, the reectivity rises to about 30 per cent. It may be that the evaporated material does not deposit in a dense structure but is somewhat porous, and when moisture is allowed to come into contact with the evaporated fllm, molecules of water are attracted into these pores and thus the index of refraction is increased. One disadvantage of forming the evaporated film as described above is that it is deleteriously affected by water. For example, when a drop of distilled water is allowed to dry on its surface, the film beneath the drop is softened to the point where it is easily wiped otr.

Vof distilled water were observedthat films of magnesium iiuoride on spectacle crown glass, made in this way, reached a minimum reflectivity of about 30% of their former value and that the reflectivity did not change when moist air was allowed to enter the bell jar. It was observed also that when drops alllowed to dry on the above lms, they did not affect the hardness of 'the film. It is believed that this increased durability results from an action wherein the high temperature of the glass causes the molecules of magnesium fluoride to have a random Ymotion much higher than at room temperature, thus allowing the molecules to form a denser film structure.

Although the above method, as heretofore proposed and used, is a satisfactory one for producing a durable magnesium fluoride film, there is considerable objection to it because of the fact that the glass must be heated to such a high temperature. This makes it impractical to coat cemented doublets, and tends to produce harmful strains in the glass. In some experiments leading to the present invention it was found possible to improve the durability of an evaporated nlm, without the use of such heat, by increasing the velocity of the molecules of evaporated material. If Vthese molecules are considerably increased in velocity, they will have a greater impact upon the glass and upon the previously evaporated molecules, and will cause the structure to become more dense. In accordance with the foregoing experiment, magnesium'iiuoride molecules which had thus been increased in velocity were allowed to strike a 'f glass .to build a lm .1; *I t A 4 in' Films made in this way were tested for water solubility and found to be more resistant than lms made in the normal way at room temperature.

Although the above increased velocity method gave some improvement in the durability of magnesium fluoride films, it soon became apparent that it did not offer a complete solution to the problem. Experiments were made also with ionic bombardment. A magnesium fluoride film was bombarded with ionized gas molecules by allowing a small amount of air to enter the bell jar following the deposition of the film by thermal evaporation, and applying a high voltage between two electrodes in the jar. While ionic and molecular bombardment improved film durability, it was found that electron bombardment offered a further improvement. Electrons can be emitted and controlled in a vacuum and a considerable amount of kinetic energy can be drectedto the glass or nlm surface. The problem was one of producing a suilicient quantity of electrons in the bell jar at a pressure of from .1 to .2 micron and directing these onto the glass to be coated. After many experiments with different types of cathodes and different shaped anodes. an electron gun, shown in Figure 2. was built and used in an arrangement shown in Figure 1 for testing the effect of electron bombardment upon films such as evaporated magnesium fluoride.

Apparatus for electronic bombardment In Figure 2, the electron gun l comprises a tantalum cathode 5 made of pure tantalum ribbon .005" thick by .090" wide. A rectangular cathode surface 0.125"x.090" forms the electron emitter. A molybdenum disc 1, centrally apertured, surrounds but does not touch the rectangular portion of the cathode. A circular metallic disc 8, 11;" thick. includes a central aperture in diameter. This acts as an electron condenser and causes the electrons leaving the cathode to form a slightly diverging bundle. The cylindrical electrodes 2 and l are first and second anodes, respectively, which are .9" in diameter. The anode 2 has a circular opening rs in diameter in the end nearest the cathode 5; a bundle of electrons passes through the opening without striking the edges. The cylindrical electrode 3 is a focusing electrode which is kept at a desired potential between that of the anodes 2 or 4 and the cathode 5, by means of a potentiometer II. The three cylindrical electrodes 2, 3, and 4 form an electrostatic lens, the focal length of which depends upon the voltages applied. The

rst anode 2 is spaced about V8" from the nearest face of the apertured plate 6. A transformer I supplies current to heat the cathode. The transformer 9 applies to the -anodes 2, 3, and 4 the high voltage which accelerates the electrons'. The high potential terminal is also connected to the metal support plate I9 as a target anode. although a separate electrode near the optical element I8 may be used. For the tests, AfC. voltages from 2,000 volts to 6,000 volts were used. While the electron. gun is shown about full size. the support plate I9 diameter and its distance 'from the gun are larger than shown,v.as indicated by broken lines. `The electron. gun acts as a half-wave rectifier and projects a beam of electrons, preferably perpendicularly. to the lens plate during each half cycle. The electrode structures 6 and 1 arev connected through shell I3 to an intermediate point on the circuit energizing the cathode. and to the low potential side of the high voltage secondary of transformer 9.

Figure 4 shows curves A or anode current vs. anode volts and B of anode current vs. cathode current for the electron gun shown in'Figure 1. Curve A was taken by holding the cathode current at 28.5 amperes and varying the anode voltage. Curve B was taken by holding the anode voltage at 5,000 volts A. C. and varying the cathode current.

Tests of films bombarded electronically Referring to Figure 1, the electron gun I described above was mounted on base I0 in position for use in a bell jar I2. Tests were made to determine the eiect of bombarding a'magnesium fluoride film with electrons during evaporation from a boat or cup Il. The vacuum in the bell jarwas brought to about .1 micron by means of a vacuum pump, not shown. Ionization apparatus, notl shown, may be included. The magnesium fluoride was evaporated from a seamless molybdenum cup Il wound with molybdenum wire I6 insulated with an aludum cement. The coating, deposited upon an optical wedge I8, was controlled by observing-the light beam from source 20 reflected from the lower surface to be coated through a nlter 22, into a photocell and electron multiplier 24. The current from the electron multiplier was read on an ultra-sensitive D. C. meter. The meter was setto read 100 per cent (full scale) for the light from the uncoated glass surface.- The electrons from the gun were focused to a spot about i" in diameter on the optical article or glass wedge I8 through opening 29 in plate It.' The voltage on the'anode was 4.000 volts A. C. and the cathode current was 62 milliamperes. rectified A. C. Magnesium fluoride was evaporated on the glass at a rate auch as to require vabout 1.5 minutes for a V4 wavelength film. -During the evaporation, the glass -and film were bombarded for one second every sixth second. The total bombarding time was, therefore, fifteen seconds. The reflectivity dropped from 100 per cent to a minimum of 35 per cent during the evaporation and bombard-v ment. When air was let into the Jar, the reflection rose to 37 per cent. Upon inspection of the film it appeared to have a non-uniform brown deposit over the surface. This absorbed some of the incident light and appeared bright and metallic by reflection. When the glass alone was bombarded with electrons there was no deposit. Apparently the electron bombardment caused a portion of the magnesium fluoride to decompose. leaving metallic magnesium on the surface of the lm.

During the next test, magnesium fluoride was evaporated on the glass until the reflection was minimum and then it was bombarded for one second every fifteen seconds for twelve times, The spot size, the anode voltage and the electron current were the same as in the first experiment. The minimum reflectivity' was 18 per cent before the bombardment started. As the bombardment progressed, the reflectivity dropped to 15 per cent and then rose to 40 per cent. When the air was let in, the reading remained at 40 per cent. Before the film was bombarded it was normal in color and free from absorption. As it was bombarded, a brownish deposit appeared in spots which seemed to be at the most intense part of the beam. When it was taken from the jar, shortly after the bombardment, the glass was just barely warm.

By inspecting the last film, it was apparent that some areas where the intensity of the electron beam was below some definite value, the material had not decomposed and the film was cleai and transparent. Drops of distilled water were placed on the film in areas where the magnesium fluoride had not decomposed and also in areas where it had. It was allowed to dry and then tested. In the areas where the intensity had been just below that required for decomposition, the water drops had almost no effect on the film. An extremely fine line was apparent at the edge of each drop, but no appreciable amount of material came ofi. This represents a major improvement over normal magnesium fluoride lms evaporated at room temperature and not bombarded. Such films are removed completely in the areas covered by distilled water drops when these drops are allowed to dry. By placing distilled water drops over areas which had been decomposed, it was observed that the film was less durable and part of it came of! in the water. Further tests showed that electronic bombardment, either during evaporation or after film had been deposited and in the same vacuum, resulted in a major improvement in hardness. The hardness was also found to be greatest for electron intensities just under the value required for decomposition of the magnesium fluoride.

Having thus proved that electronic bombardment increases the mechanical durability and decreases the action of water on magnesium fluoride films, tests were made to determine how this effect varies with voltage. current, time, temperature, etc. The A.C. transformer 9 of Figure 1 was replaced by a D.C. power supply capable of delivering voltages from 500 volts to 7,000 volts, and capable of delivering current as high as l/i ampere.

In order to make reliable comparisonsbetween bombarded and unbombarded films of magnesium fluoride, the arrangement shown in Figure 1 was adopted. Wedges I8 made of spectacle crown glass were placed successively over a circular hole in plate I9 in different tests. A vertical mask or vane 2l shielded a portion of th glass from the electron stream projected from the gun I, casting an electron shadow as shown in Figure 3 to left of line 23--23. The complete circular surface portion of glass exposed by the hole in plate I9 was visible from the evaporating boat I4 and was therefore coated with a uniform layer of magnesiur fluoride.

In order to test for improvement in water resistance, the wedge was placed in distilled water to a level about half the height of the wedge, as shown by broken line 25--25 and left in the water for 24 hours. It was found that the shaded area (not bombarded) to left of line 23-23 and below line 25-25, came ofi in the water in about an hour, while the rest of the film was still in good condition. To test for hardness, a pencil eraser was rubbed across the upper coated portion of the wedge above line 25-25 shown in Figure 3. The bombarded and unbombarded areas were given the same treatment by passing the eraser across the line of demarcation 23-23. It was found that the unbombarded areas above 25-25 to left of line 23-23- scratched and eventually rubbed off with the eraser, while the bombarded area was so hard that it was not eroded or scratched by the eraser.

It was observed from the first test wedges that bombarding of magnesium fluoride films with electrons reduced the thickness of the films. This is easy to detect because the color of the bombarded area as observed by the eye is different from the unbombarded area, the shift being in the direction of the red end of the spectrum. Thus, when the wedge shown in Figure 3 is observed when taken from the jar in Figure l, the segment to left of 23-23 would normally be bluepurple by reflection and the right side segment would be red-purple. This change in color provides a means of measuring the amount of compacting of the film produced by bombardment after evaporation', but in the same vacuum.

Referring to the arrangement shown in Figure 1, an ultra-'violet filter 22 is placed in front of the photo-cell and electron multiplier. The film of magnesium fluoride is evaporated to such thickness as to reflect minimum green light. But when using an ultra-violet filter (3,650 A.) the reflectivity drops during evaporation from per cent of its former value down to about 14 per cent,

\ the results of these tests.

is definite proof that bombardment produces a more compact structure. It was observed also that when a normal unbombarded film of magnesium fluoride is brought into contact with air containing moisture, the reflectivity rises from 50 per cent to 68 per cent. This indicates that moisture is attracted into the porous structure of a normal film and causes it to swell, thus increasing its thickness, reducing its mechanical strength,

. and increasing its solubility in water.

By using the method described above, it was possible to study the effect of, changing various factors in the bombarding process. For instance, the bombarding time was varied over a range from eight minutes down to one-quarter second, keeping the product of intensity and time constant for each test. Readings of reectivity were taken at various intervals during the bombarding process for each test. Figure shows Curve C is for atotal bombarding time of eight minutes. Curve D is for a total time of two minutes. Curve E is for a total time of thirty seconds, and curve F is for a total time of six seconds. A single reading was taken at one-quarter second, and correspondingly high intensity. In this case, the reflection dropped from 50 per cent to 42 per cent.

Figure 5 shows that the greatest reduction in film thickness occurs when the exposure time is in the range from two minutes to thirty seconds.

The total exposure was 175 watt-seconds per square inch for each of the above tests.

The test wedges which were bombarded for eight minutes and for two minutes showed no absorption. The intensity for these exposure times was, therefore, low enough to avoid chemical breakdown of the magnesium fluoride. The wedge which was bombarded for thirty seconds showed just a perceptible amount of absorption. The six-seconds test was quite dark, indicating a considerable amount of disintegration. All of the test films with the exception of the one quarter second test were very hard and quite resistant to water. 'Ihe one-quarter second film was quite soft and rubbed oil. with a pencil eraser.

It was observed that bombarded films which showed just a perceptible amount of absorption when taken from the jar, cleared up in a few hours and showed no absorption. It is assumed that the slight amount of metallic magnesium resulting from distintegration, later combined with oxygen in the air to form transparent magnesium oxide.

The effect of the voltage of the bombarding electrons was studied for applied potentials of 500 volts, 2,000 volts, 3,500 volts, 4,000 volts and 5,500 volts. showed no improvement in film hardness and water solubility, even though the lm was bombarded with an electron energy of 204 watt-seconds per square inch, and an exposure time of 180 seconds. When the anode voltage was 2,000 volts and the electron energy was 360 watt-seconds per square inch,-there wasa definite improvement in hardness and a reduction in the effect of water on the film. The hardness and durability continued to improve for 3,500 and 4,000 volts. At 5,500 volts, the hardness was only slightly greater than for 4,000 volts. It was concluded that most of the desirable effect of electron bombardment of $41 wavelength films of magnesium fluoride could be obtained with 5,000 volt electrons. The exposure, or product of intensity and timel should be just under the value required to cause chemical breakdown and an exposure time in the range between two minutes and a half minute was found to be quite satisfactory.

After many tests, it was concluded that disintegration of the magnesium fluoride molecules is due to the combined action of electron bombardment and surface temperature. In order to prove this point, a. magnesium uoride film was bombarded with an electron exposure (product of time and intensity) just under that required for disintegration. The test wedge was then allowed to cool in the vacuum for fifteen minutes, and it was then given another dose of electron bombardment equal to the first. The glass then cooled for fifteen minutes more in the vacuum and a third dose was given. When the test wedge was taken from the jar, the film had just a perceptible amount of absorption and this cleared up in less than an hour. Previous tests showed that if this intensity of bombardment had been applied in a single dos'e of three times the duration of those given above, or, if three times the intensity had been applied for the same time, the film would have been badly darkened by disintegration.

It might at first be supposed that the hardening action on magnesium fluoride films produced by electron bombardment is produced by temperature, and that the electrons merely serve to heat the surface of the glass. It can be shown that this is not the case, and that electron bombardment acts in a different way to produce durable films. It is believed that this action is produced by the impact of the electrons upon the mole- -cules of the film and by known ability of electrons to penetrate beyond the surface of a material.

The surface temperature developed by a fixed rate of energy absorption, and the temperature distribution within a semi-infinite solid were cal- ,culated by the method of Fishenden and Saunders as outlined in Applied Mathematics in A test with 500 volt electrons Chemical Engineering" (pgs. 241-245) by Sherwood and Reed. It was found that the surface temperature:-

t=0.208 W K- CP (1) End 2KA9 FV W (2) where i t=surface temperature C.

W=energy entering the surface in watts/sq, cm.

A6=the chosen increment of time in seconds.

AX=the corresponding increment of distance from the surface in cm. (calculated from (2) K=thermal conductivity in calories cm./sec./sq.

cm./ C.

C=specic heat in ca1ories/gram/ C.

P=density in grams/cu. cm.

For crown glass this becomes:

=5.43 WV A9 in oC. (3)

and

AX :0.1094`1 in cm. (4)

Figure 6 shows curves of temperature distribution in a semi-infinite solid of crown glass when its surface is bombarded at the rate of wattseconds per square inch. The curves for bombardment times of six seconds, thirty seconds and 120 seconds are shown. These bombardment times and the rate of bombardment are the same as thre of the tests described earlier and shown in Figure 5. It will be seen from Figure 6 that the surface temperature rise for six seconds bombardment is 'Il' C., referring to curve K. For thirty seconds, curve L, the temperature rise. is 31.7 C. and for 120 seconds. curve M, the temperature rise is 15.8 C. Although these curves are for a semi-inilnite solid, 'the values of surface temperature would not be appreciably diii'erent for a crown lens having a thickness of 1 cm.

It is quite apparent from the above curves that the surface temperature produced by the electron bombardment values given is not suiiiciently high to produce any appreciable hardening due to temperature alone. The high temperature method of hardening magnesium iluoride tllms in common use at the present time calls for heating the glass to about 225 C. prior to evaporating the film of magnesium fluoride. For the case of a l2" diameter bell jar and a lens plate 7%" diameter, this method requires about 600 watts of radiant heat-applied for about fifteen minutes. This amounts to 13,500 watt-seconds per square inch as compared to about 350 watt-seconds per square inch required to bombard electronically both surfaces of a plate of glass about 0.1" thick. If a crown glass lens 1 cm. thick is bombarded on each surface for thirty seconds at a rate of 175 wattseconds/sq. in., the temperature of the lens would rise by less than 30 C. This is low enough to make it quite practical to apply the electronic hardening method to cemented doublets. The hardness of magnesium fiuoride lms l(given the treatment described above) is at least equal to, and possiblyv greater than, that produced by the high temperature method.

In accordance with the above disclosed method -of increasing the durability of magnesium iluoride iilms by bombarding them with electrons in the same vacuum in which the films are evaporated, it is pointed out that for maximum effect the electron exposure should be just .under thel value required to cause disintegration of the magnesium fluoride. The eilcient application of this method requires that a sizable lens plate carry a plurality of optical objects to be coated and bombarded uniformly with electrons. II it were practical to project a continuous beam of electrons of suillcient intensity and uniformity over the whole area of the lens plate, this would be a desirable way of bombarding the lens surfaces. It is, however, very diiicult to project a uniform electron beam of desired characteristics over a large area.

Electron bombardment by scanning evaporated films. Referring to Figure 9, a hermetically-sealed cylindrical shell 30 contains the slow-speed horizontal deecting coil 32 and the high-speed verticaP deilecting coil 34. A focusing coil 36 is enclosed in a hermeticaliy-sealed cylindrical shell as and is surrounded by an iron shell 40, except for a small gap 42. The deiiecting coils are similar to those described in Patent 2,167,379, by W. A. Toison. in Nicolson 1,470,696 may be used. or mechanical means may be employed forscanning, as by oscillating the electron gun bodily in one plane while it is moved in a direction at right angles more slowly.

The anode apertured disc 44 is preferably made of molybdenum and has a circular hole in the center. The apertured member 6 is made 0f stainless non-magnetic steel, and has a diameter hole in the center. Disc I is similar in nature to disc 1 in Figure 2. The member 6, which may be .116" thick, acts as an electron condenser for the electrons emitted from the cathode 5. The cathode 5 is made of .090" x .005" tantalum ribbon and presents a rectangular area 1/8" x .090". The cathode support structure 46, apertured structure 6, and disc I are held in a brass shell I3 and are insulated from the anode 44 by the isolantite insulators 48. The deecting coils 32 and 34, the focusing coil 36 and the apertured anode 44 are mounted in a copper shell 50 which helps to conduct the heat down a pair of copper feet 52 (Figure 8) which are fastened to the metallic base plate I0 in the bellljar.

In operation, the electrons emitted' from the tantalum cathode are concentrated by the aperture of the member 3 to such extent that they pass through the aperture of anode 44 and 2 without hitting it. The magnetic focusing coil 36 will then focus an image of the cathode or the image of the virtual "cross-over upon the lens plate I9 in Figure 8. The image of the virtual cross-over is considerably smaller and more concentrated than the image of the cathode.

Figure 8 shows how the electron gun 28 is mounted with respect to the evaporating boat I4 and the lens plate I9. The electron beam is caused to scan the entire lens plate several times per second for the required length of time. This is accomplished by connecting the deecting coils of the gun to a pair of saw-tooth oscillators, the circuits of which are shown in Figure 10. In order to prevent "streaks," it is desirable to impress a saw-tooth frequency of 400 cycles/sec. or higher on one set of defiecting coils and a sawtooth frequency of from two to ten cycles per second on the other set of defiecting coils. The diameter of the electron spot at the lens plate was about 1A" in the test apparatus. It is not easy to keep the electron image in good focus all over the whole lens plate in view of the different angles involved, but this is of little practical lmportance so long as the plane of plate I9 is substantially normal to the mean electron path. as shown in the inclined position I 9'. The support plate is iirst positioned as shown at'IS for receiving the evaporated coating from boat I4 which is positioned In coaxial relation with I9. After the coating has been accomplished, the plate is rotated about its axis of support 26 to position I9' in perpendicular relation to the axis of the electron gun 28. Theglass parts are bombarded with electrons as described above. The plate is then rotated to a position of relative to position I9 to coat the other sides of the optical articles, thence to a position 180 from position I9' for bombardment. Alternatively, both' sides of the articles may be coated in succession, followed by bombardment.v These movements may be controlled by magnetic means disposed Within and without the jar (not shown).

Spiral scanning as The bombardlng exposure necessary for the desired hardening effect isa function-ofthe material of the optical body, the thickness of the body. It has been found that a half wavelength coating requires a different treatment than does a quarter wavelength coating. For example, a 1/2 `wave-.

length coating has been hardened by steps. de-

positing a V4 wavelength coating followed by electron bombardment. then depositing the remaining V4 wavelength coating. followed by bombardment.

' Forming a 1/2 wavelength coating in one step, folintensity will be reduced-but the time of bombarding any pointis increased and the total number of electrons falling on any given area during the total time will be practically the same. -In practicing the invention, the desired hardness and durability of illm has been eiTected by electron bombardment in less than ten minutes. 'I'he exposure, therefore. remains substantially uniform. It is desirable that the power and kinetic energy in the electron beam be constant and not be changed by the deflectlng means.

The gun shown in Figure 9 meets this requirement. This electron gun will project more than 1,000 watts in an electron beam 1/4" or less in diameter when the potential difference is 5.000 volts. As a matter of comparison, a normal cathode ray tube used in television uses a beam of around watts power. Preferably, the cathode is made negative 5,000 volts and the anode is at ground potential. The beam from the above gun was also used for evaporating the refractory materials thorium oxide, aluminum oxide, and silicon carbide from crucibles by focusing the beam upon the surface of the material without melting the remainder thereof. One-quarter wavelength films of silicon carbide made in this manner were extremely hard and durable. They reflected about 50% of the incident light. Evaporation by this means has advantages such as flexibility of control.

In case it is desiredto evaporate materials from different boats or crucibles in some desired sequence, as in depositing multi-layer coatings in my application Ser. 470,583, led December 30, 1942, the beam fromv a gun of this nature may be electrically or magnetically directed from one to tion, has been disclosed for treating films forme'd menting. This increases the oost and cuts down the number of finished lenses per bell iar load.

2. The lower temperature resulting from the electronic process produces much less strain in the glass. After lenses or prisms have been carefully ground and polished to very precise curved or plane surfaces,` it is a very decided disadvantage to have to heat these optics to 225 C. and cool them rapidly. This temperature may even cause a slight change in the index of refraction of some glasses.

3. The electronic process does not interfere with obtaining a satisfactory vacuum in the shortest possible time. When the metal and glass parts inside the bell jar are heated to 225 C. .they give off much-gas and it requires more time to obtain a satisfactory vacuum. This materially increases the cost. It is usually necessary to make a compromise between the degree of vacuum and the pumping time. If the compromise is in the direction of poorer vacuums. then the film will be softer and part of the gain in hardness possible with the high temperature method is sacrificed.

4. The electronic method of hardening A wavelength magnesium fluoride fllms may be applied either during evaporation or after the film has been completed, without appreciably affecting the result. The only requirement is that the treatment be applied in the same vacuum in which the evaporation is done. It is simpler. from the operating standpoint. toapply the hardening process lafter the 111m is completed. The operator coats the glass parts in the normal way and then presses a button. which starts the bombardanother of the crucibles for'bombardment and f evaporation of the materials therein in the order andtime or intensity desired. Such a composite coating may be bombarded, after forming. or each layer may be bombarded during or after forming. Similarly, the electron beam from theguns of Figures 2 or 9 may be selectively directed in any desired manner among the optical articles being treated. For example, one lens may require more bombardment than the rest. This can be accompiished by orienting the gun or-electrically deilecting the beam, and adjusting voltage if necessary, in order to selectively bombard the selected area to the exclusion of others. i

Electron bombardment may, therefore, be used first for cleaning the optical surface, and nally for treating the nlm. While bombardment, according toa preferred embodiment.i 91 ille invenment and allows it to continue for a fixed time. This can be done automaticallyl The glass is then turned over by magnetic or other means and the process is repeated. No further baking is required after the glass is taken from the jar. The `glass is not too hot to handle and may be immediately taken from the mounts. This saves cost of more mounts.

5. Electronic bombardment is a very excellent means of cleaning glass just prior to coating. The use of ionic bombardment of glass for cleaning is well-known, and generally used. The gas in the bell jar is ionized by applying several thousand volts between electrodes during the normal pumping cycle. The disadvantage of this method is that ionic bombardment stops as soon as a dark vacuum is obtained. This occurs at about 2 microns. and it ordinarily takes from five to ten minutes or more to pump the -jar down from way of cleaning oil vapor or other contamination from the glass.

6. Electronic bombardment is useful 'in treat ing non-reflecting lms deposited by thermal evaporation upon plastic lenses and the like, made for example of Cellulold, "Cellophane" or Luciteff or similar materials that would be dele- 13 teriously aifected by heat necessary to harden the film.

7. Electron bombardmentv in accordance with the invention has a similar advantage, when used to harden and make more durable a half-wave coating of magnesium fluoride or the like, on front-surface optical mirrors of thermally deposited aluminum or silver.

Other advantages and uses will become apparent to those skilled in the art. While preferred embodiments ofthe invention have been shown, it will be apparent that the invention is not limited to such embodiments.

I claim as my invention:

1. In apparatus for. producing a thin durable layer upon an optical surface of a body, an evacuated chamber arranged to removably receive said body, means for evacuating said chamber, means for adjustably supporting said body in said chamber. thermal means for evaporating material to form said layer, and an electron gun supported within said chamber and arranged for directing a stream of relatively high velocity electrons upon said layer to harden said layer, said electron gun being provided with means for focusing and means for causing said stream of electrons to scan said surface with substantially uniform electron exposure.

2. In apparatus for producing a thin durable' coating` upon selected surfaces of a plurality of optical bodies, respectively, an evacuated chamber, means for evacuating'said chamber, means 'for removably supporting said bodies to expose said surfaces within said chamber, means for applying material to form said coating simultaneously on said surfaces, and means for subjecting said coatings successively to bombardment by an electron discharge with substantially Aequal exposure of said selected surfaces for improving the hardness and durability of said coatings.

3. In apparatus for producing a thin hardened coating on surfaces of optical bodies, an evacuated chamber including a bell jar and base, means for evacuating said chamber, a lens plate carrying said bodies with surfaces exposed within said chamber, thermal evaporating means within said chamber for applying reiiection reducing coating material upon exposed surfaces of said bodies, an electron gun mounted within said 14 chamber in laterally spaced relation for bombarding said bodies with relatively high velocity electrons of such exposure as to substantially enhance the durability of said coating, means for mounting said plate within said chamber for transverse rotation from a position wherein a perpendicular through the center of said plate is substantially in line with said evaporating means to a position wherein said plate is perpendicular to the axis of said electron gun, for subjecting the several surfaces to substantially the same coating and electron exposure, respectively.

4. In apparatus for producing a thin durable coating on a surface of an optical body an evacuated chamber, means for evacuating said chamber, support means carrying said optical body and exposing said surface within said chamber, thermal evaporating means within said chamber for` applying material to form said coating upon said exposed surface, electron projecting means for bombarding said coating with electrons, and mounted within said chamber in spaced relation to said evaporating means, whereby said evaporating material and said electrons arrive at said surface at relatively diierent angles, and means for mounting said support means for transverse rotation, whereby said support may be rotated so as to assume any desired position with relation to said evaporating means and said electron projecting means.

. GLENN L. DIMMICK.

REFERENCES crTan The following references are of record in the le of this patent:

UNITED STATES PATENTS Date 

